This invention is generally related to loudspeakers, and more particularly to a system and method for modifying frequency response characteristics of a loudspeaker that is placed near a wall or other acoustically-reflective surface.
As is well known, a loudspeaker receives an electrical signal representing an audio sound and converts the electrical signal to an audio sound wave via a loudspeaker driver unit. The driver unit usually comprises an electro-magnetic motor which responds to an electrical signal to move a diaphragm. The frequencies and nature of the electrical signal control the frequencies and nature of the audio signal. Other types of driver units are available and commonly known in the art.
The loudspeaker typically comprises a driver unit and an enclosure. The driver unit acts as set forth above to generate the audio wave. The enclosure acts to suspend the driver unit as desired and contain the sound wave generated by the driver unit on the rear side.
The quality of the audio reproduction by an individual loudspeaker is influenced by the interaction of the produced audio waves and the nearby boundary surfaces such as walls. The loudspeaker performance is particularly influenced by the interaction between the produced audio waves and nearby walls. A reduction in loudspeaker performance quality occurs because sonic reflections from the nearby walls interfere with sonic waves emanating from the loudspeaker, often causing a series of frequencies to be partially cancelled and another series of frequencies to be partially reinforced. The first frequency of each series is affected the most severely and has the most substantial negative effects on the performance of the loudspeaker. Primary partial cancellation occurs when the central axis of the driver unit is spaced from the flat surface a distance that is equal to xc2xc wavelength of the audio wave so that the reflected audio wave is delayed approximately xc2xd wavelength or xe2x80x9cout of phasexe2x80x9dwhen it returns to the location of the diaphragm. Primary partial reinforcement occurs when the driver unit is spaced from the reflecting surface a distance that is equal to xc2xd wavelength of the audio wave. Then, the reflected audio wave is approximately 1 wavelength delayed, or xe2x80x9cin phasexe2x80x9d when it returns to the location of the diaphragm.
Since the velocity of audio waves in air is approximately 345 meters/second, the frequency at which the original audio wave will be canceled, Fc, is equal to 86/D, where D is the distance between the nearby wall and the driver unit center (measured in meters). The frequency at which the original audio wave will be reinforced, Fr, is equal to 172/D. As the distance D decreases, the higher the cancellation frequency and the reinforcing frequency become. For example, if the loudspeaker diaphragm to wall distance is 17 cm, then the frequency range of partial cancellation will be centered at approximately 500 Hz, and the frequency range of the reinforcement will be centered at approximately 1 kHz.
Of course, as the separation distance between a speaker and a wall (or other reflective surface) increases, the affect of the reflective interference is reduced, due to the amplitude attenuation of the sound wave. However, as speakers are placed in close proximity to a wall or other sound reflective surface (as they often are), then the deleterious impact may become more noticeable.
As is known, many loudspeakers are engineered to exhibit a frequency response that has desired characteristics when placed in a room at a significant distance from room walls. If such a loudspeaker is placed in close proximity to a wall, or two walls in the case of placement near a corner of the room, the frequency response, as measured by the sound pressure level produced at a distance from the speaker, will change from that produced when positioned farther from the wall.
The frequency response changes at lower frequencies (below approximately 300 Hertz) are mainly due to two effects. The first effect is that the wall tends to confine the radiation from the loudspeaker to a smaller solid angle and therefore to increase the sonic pressure produced by the loudspeaker below a certain frequency. The second effect is that sonic reflection from the wall causes partial cancellation of some frequencies. The degree of both these effects, and the frequencies affected by the second effect are dependent on the speaker""s distance from the wall.
In view of the foregoing, it is desired to provide an audio system that has improved performance when speakers are positioned near a wall or other acoustically-reflective surface.
The present invention is directed to a speaker system that more accurately reproduces audio sounds by minimizing the negative effects from sound reflecting from a nearby wall. The invention is embodied in both methods and apparatus of differing forms. In one embodiment, the invention comprises a method that receives an electrical signal embodying an audio signal to be communicated to the speaker, and modifies the electrical signal by a measure that is based upon a separation distance separating the speaker and a wall. In another embodiment, the invention comprises a driver circuit for a an audio amplifier that, in turn, drives a speaker. The driver circuit includes a receiver configured to receive an electrical signal embodying an audio signal to be communicated to the speaker, and a signal modifying circuit configured to modify the electrical signal by a measure that is based upon a separation distance separating the speaker and a wall.